In recent years, vertically retractable shutters have been widely used in houses and garages, and a number of electrically driven shutters increases sharply. A shutter control apparatus for opening and closing a shutter integrates a safety function in addition to a control function. The safety function includes a mechanism that halts extension of the shutter (closing), in particular, when an obstacle is detected, or restricts a driving force.
In a conventional manner, this safety function has entailed a dedicated control circuit and switches, which has complicated an entire mechanism and increased the cost. A simple output-torque restriction function free from a control circuit, i.e. a torque transmitter integrating a torque limiter has been thus demanded from the market.
A conventional mechanical torque limiter disclosed in the Japanese Patent Application Examined Publication No. S55-21893 teaches as follows:
Each rotary body of a driver and a follower faces each other. The respective rotary bodies made of cylindrical boss member and having a flange are mounted on a driving shaft and a following shaft respectively. Both the shafts are disposed co-axially. One of two rotary bodies has angular grooves in the axial direction on its flange, and another has U-shaped grooves on its flange. A roller is disposed between the angular grooves and U-shaped grooves, which is urged to the U-shaped grooves by a torque spring.
A threshold-torque-value is set by adjusting a spring pressure. When a load over the threshold-torque-value is imposed on the torque limiter, the roller reacts the elastic force of the torque spring, and comes off the U-shaped groove. As a result, torque transmission is discontinued.
Another torque limiter has an inner rotary body, an outer rotary body and a roller in between. A spring urges the roller, which comes off the U-shaped groove, then the torque transmission is discontinued.
These conventional structures transmit the torque by engaging a roller between angular grooves and U-shaped grooves provided respectively on a flange of one rotary body and a flange of another rotary body. The roller is come off the U-shaped groove by excessive load, then torque transmission is discontinued. A limit torque is thus constant regardless of rotating directions.
When an electric shutter employs a torque transmitter including the conventional torque limiter discussed above and a motor with reduction gears as a driver, a user encounters the following problem. Assume that the torque has been adjusted to meet the condition of shutter extension (closing), i.e. detection of an obstacle at shutter extension loads an excessive torque on the torque limiter, which discontinues the torque transmission. Then the torque limiter happens to be activated at shutter retraction (opening). Therefore, the shutter retraction is sometimes bothered.
Further, when an article is caught in the electric shutter, the motor is kept driving and the torque limiter is kept working until the current to the motor is cut off. In this case, torque is varied, or noise from the torque limiter occurs, thereby displeasing people around there. Since the torque limiter is kept working in this case, a mechanical sliding section is worn out and its service life is shortened.
In order to avoid the problems discussed above, limit switches are provided in the torque limiter at upper and lower limits of traveling the electric shutter, thereby halting the motor operation.
Safety-oriented users demand a torque transmitter, which can be halted electrically in addition to its mechanical torque limiter.
Another torque limiter is disclosed in the U.S. Pat. No. 4,792,321 where a shape of teeth to which balls are urged is formed symmetrically so that torque limiter can function in both of forward and reverse directions of a rotary shaft. In this case, both the directions need an equal limit torque. This prior art also discloses a torque limiter of which teeth are asymmetric and have a shoulder preventing movement of a ball from a ball-receiving recess in one direction of input shaft so that a unidirectional torque limit and a one-way brake are provided. In this case, the torque limiter functions only in one given direction.
And yet, the structure of this prior art does not discontinue the torque transmission, but a torque value close to the limit torque is kept on transmitting. The transmitted torque is changed every time the ball climbs over the teeth, and it does not remain stable. Therefore, when the torque limiter functions, noise and vibration occur.